The present invention pertains generally to immobilizing particulate matter contained in a "packed" bed reactor so as to prevent powder migration, compaction, coalescence, or the like. More specifically, this invention relates to a technique for immobilizing particulate materials using a microporous foam-like polymer such that a) the particulate retains its essential chemical nature, h) the local movement of the particulate particles is not unduly restricted, c) bulk powder migration and is prevented, d) physical and chemical access to the particulate is unchanged over time, and e) very high particulate densities are achieved.
The utility of many materials used in industrial applications, particularly catalysts, is often limited by the surface area of the material available for reaction at any particular time. High surface forms of these materials, typically finely divided powders, are used extensively but efficiently using them in commercial processes remains of primary concern due to the need for bringing reactants together and constantly replenishing reagents as reactions proceed.
Increasing the reaction surface of a material is, therefore, only a first step. In order to use a material in reaction, it must be brought into contact, or at least very close proximity, with other active agents in order for a reaction to proceed. Unfortunately, finely divided powders are often difficult to handle in a manner which easily admits to intimate mixing. Various methods to address this problems have been implemented over the years. Some of the more familiar techniques include fluidized beds, and packed "fixed," beds wherein the reactive agent is supported by a secondary material or loosely contained in a closed retorts., and packed "moving" beds wherein reagents are mechanically passed through a reaction.
One very important class of materials used in packed beds are materials which form stable compounds with various gaseous species, i.e., getters, scrubbers, filters, converters (as in combustion exhaust converters), and the like. In particular, the class of materials known as hydrogen occulders, materials which form reversible "hydride" compounds when exposed to ambient hydrogen gas, are materials seen as the key to a hydrogen based economy. The ability to safely store very large quantities of this gas and quickly retrieve it for use would be a first step in replacing gasoline with hydrogen as the primary energy source for automobile transportation.
Such materials are known to include transition metal alloys having the formula A.sub.x B.sub.y C.sub.z where A is Fe, Mg, La, or Zr, where B is Co, Cr, Cu, Ni, or Ti and where C is Al. The parameter x in these alloys may vary from about 0.1 to about 2, y may vary between about 0.1 to about 5 and z may vary from about 0 to about 3. In particular, alloys such as those based on Fe--Ti and La--Ni--Al are often used in these systems. The lattice of these materials, however, are known to undergo significant volumetric deformation when the hydride reaction take place. The alloy lattice "swells," with the introduction of hydrogen, which fractures alloy particles breaking them up into still smaller particles. This size reduction promotes settling of the particles which eventually results in significant compaction and consolidation in the bed. As these particulate settle their local movement becomes increasingly restricted until the local swelling of many individual particles sums to create macroscopic swelling in the bed itself. In extreme, but not unusual, cases swelling of this sort will eventually split the containment vessel. Controlling the movement of the powder "fines" generated by these systems, therefore, is of primary concern.
Hybride reaction containers are often designed with elaborate internal baffling and channeling structures to prevent such powder migration. These techniques have worked well but they are expensive, costing not only money due to the complexity of the vessel fabrication, but decreased efficiency due to a lower packing density. A method for "fixing" hybride particles, immobilizing them in such a way as to prevent their compaction, would provide significant relief in reactor design.
While hydride materials have been emphasized for illustrative purposes similar problems occur in other types of packed beds whether or not particle comminution takes place during gas cycling: it is well known that the movement of pressure pulses or "waves" propagating through a bed can and does perturb the particulate contained therein sufficiently to tightly pack it the point where it blocks further gas transport. While this compaction may not lead to vessel failure (no swelling) it none-the less destroys integrity and effectiveness of the bed by restricting flow of a reacting fluid, be it a gas or a liquid.
Finally, catalytic reactors, such as petroleum "crackers", NO.sub.x converters, as well as hybride reactors, can be highly exothermic. Controlling the release of this waste heat in order to prevent it from damaging both the bed and surrounding equipment can be difficult. By selectively introducing an immobilizing medium whose thermal properties are complimentary with those of the bed, in use, the excess heat generated by the bed reaction can be exactly moderated by a phase transition heat of reaction, such as a glass transition, in the immobilizing media. As the bed reaction heats the media, the media passively uses that heat to initiate a reversible phase transition, and acts to "sink" the heat of reaction in the bed and thereby eliminating "hot spots."